Rolling bearing and belt continuously variable transmission

ABSTRACT

A rolling bearing has the radius of curvature of the raceway on an inner ring and an outer ring is from not smaller than 50.1% to not greater than 51.9% of the diameter of the balls, and at least one of the compositions of the bearing is obtained by forming an alloy steel having a carbon (C) content of from not smaller than 0.50% to not greater than 0.90%, a chromium (Cr) content of from not smaller than 3.0% to not greater than 15.0%, a manganese (Mn) content of from not smaller than 0.10% to not greater than 2.0%, a silicon (Si) content of from not smaller than 0.10% to not greater than 2.0%, a molybdenum (Mo) content of zero or not greater than 2.0% by weight, and a vanadium (V) content of zero or not greater than 2.0%, by weight.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to a rolling bearing for bearing apulley shaft of a belt continuously variable transmission for vehicle.

[0003] 2. Description of the Related Art

[0004] A belt continuously variable transmission for vehicle has amechanism for continuously changing the radius of belt-driven pulley asa variable speed mechanism for automatic transmission. For example, asshown in FIG. 6, an input shaft (drive shaft) 5 and an output shaft(driven shaft) 6 which are disposed in parallel to each other areprovided with pulleys 7 and 8, respectively. A metallic belt 9 is woundround these pulleys. This belt 9 comprises two lines of ring composed ofa laminate of about 10 sheets of thin plates having a thickness of about0.2 mm and a number of thin friction pieces 92 (having a thickness ofabout 2 mm) attached thereto. The pushing force developed when thesefriction pieces 92 push each other causes the power to be transmitted.

[0005] A driving force is transmitted from the input shaft pulley(primary pulley) to the output shaft pulley (secondary pulley) 8 viathis belt 9. Both the pulleys 7, 8 are composed of fixed conical plates71, 81 fixed to the shafts 5, 6 and movable conical plates 72, 82capable of moving in the axial direction by a hydraulic mechanism,respectively. The two conical plates form a V-shaped pulley groove.

[0006] By moving the movable plates 72, 82 of the pulleys 7, 8 in theaxial direction to change the width of the groove and changing theposition at which the belt 9 comes in contact with the pulleys 7, 8, thegear ratio can be continuously changed. For example, by reducing thewidth of the groove of the input shaft pulley while increasing the widthof the output shaft pulley, the effective radius of rotation of theinput shaft pulley is reduced while the effective radius of rotation ofthe output shaft pulley is increased, obtaining a great gear ratio.

[0007] Shafts (pulley shaft) 71 a, 81 a integrated to the fixed conicalplates 71, 81 of the pulleys 7, 8 are born by radial ball bearings 11,12, respectively. These pulley shafts 71 a, 81 a are each subject tothrust load as a reaction force when they transmit the shaft output tothe following stage. Therefore, it is necessary to prevent the radialball bearings 11, 12 from being dislocated in the axial direction bythis thrust load to cause the center thereof to deviate from the inputshaft pulley to the output shaft pulley (so-called “centerdislocation”). As the aforementioned center dislocation increases, thebelt 9 meanders to cause the ring 91 and the friction piece 92 to makeinappropriate contact with each other occasionally to damage.Alternatively, the ball bearings 11, 12 can undergo slippage to generatea large amount of heat.

[0008] As a measure against this problem, JP-B-8-30526 proposes that aball bearing for bearing a pulley shaft be provided with a ratio (R/D)of radius (R) of curvature of inner and outer ring raceways to balldiameter (D) of smaller than an ordinary standard predetermined value(0.53) (50.1 to 50.9% for inner ring and 50.1 to 51.9% for outer ring).The smaller this ratio (R/D) is, the more difficultly can move the ballbearing in the axial direction under thrust load and hence the moredifficultly can occur center dislocation between the pulleys.

[0009] JP-A-10-292859 proposes the use of a four-point contact ballbearing having retained austenite in the inner ring and balls in anamount of not greater than 5%. In some detail, by predetermining theretained austenite content to not greater than 5%, the dimensionalchange due to the decomposition of retained austenite during heatgeneration is lessened.

[0010] On the other hand, as a lubricant for belt continuously variabletransmission, a traction oil (lubricant containing a special abrasionadjustor having a traction coefficient of not smaller than 0.09 and aviscosity of not smaller than 30.8 cst (30.8×10⁻⁵ m²/s) at 40° C.) isapplied to the aforementioned ball bearing for bearing pulley shaft aswell to allow torque converter, gear mechanism, hydraulic mechanism, wetclutch, etc. to operate smoothly for power transmission.

[0011] When the ratio (R/D) is reduced as in JP-B-8-30526, the contactarea of the raceway with the balls increases, and the surface pressureis reduced. Thus, it can be expected that the bearing life is prolonged.

[0012] However, it was found that a radial ball bearing lubricated witha traction oil can difficultly be provided with an expected prolongedlife merely by reducing the surface pressure. This is presumably becausethe lubricant is exfoliated in different manners from traction oil tomineral oil.

[0013] In other words, in the case where the lubricant is a mineral oil,the starting point of flaking is located at the center of the width ofthe raceway as shown in FIG. 7A. On the contrary, in the case where thelubricant is a traction oil, the starting point of flaking is mostlylocated at a site apart from the center of the width of the raceway asshown in FIG. 7B. Since the difference V in rotary speed between theraceway and the balls is a great factor, it is thought that the bearinglife is somewhat affected not only by the surface pressure developed bythe raceway and the balls but also by the difference V in rotary speed.Further, the reduction of ratio (R/D) leads to ease of the balls incoming in contact with sites apart from the center of the width of theraceway.

[0014] Accordingly, a radial ball bearing for bearing the pulley shaftof a belt continuously variable transmission cannot accomplish both thereduction of center dislocation between the two pulleys and theprolongation of bearing life merely by reducing the aforementioned ratio(R/D).

[0015] In addition to this, the short life of rolling bearings forbearing the rotary shaft of pulley of belt continuously variabletransmission is presumably attributed to the following mechanism.

[0016] In some detail, the rolling bearing for the aforementionedpurpose undergoes not only the aforementioned slippage with centerdislocation between pulleys but also slippage and vibration due to stickslip friction that occurs on the metallic belt wound round the pulleys.Thus, the lubricant film can be more easily exfoliated. As a result,this bearing is subject to heat generation and surface fatigue due tometallic contact and production of newly produced surface. The newlyproduced surface then acts as a catalyst to allow hydrocarbons or watercontent in the lubricant to be decomposed to hydrogen that specificallycauses early flaking.

SUMMARY OF THE INVENTION

[0017] The present invention has been worked out to solve these problemswith the related art technique. An aim of the present invention is toprovide a radial ball bearing for bearing the pulley shaft of a beltcontinuously variable transmission which can attain a prolonged bearinglife while lessening center dislocation between the two pulleys roundwhich the belt is wound.

[0018] In order to accomplish the aforementioned aim, the presentinvention provides a rolling bearing has an inner ring, an outer ring, aplurality of balls as rolling elements rollably interposed between theinner ring and the outer ring, wherein the radius of curvature of theraceway on the inner ring and the outer ring is from not smaller than50.1% to not greater than 51.9% of the diameter of the balls, and atleast one of the inner ring, the outer ring and the rolling elements isobtained by forming an alloy steel having, a carbon (C) content of fromnot smaller than 0.50% by weight to not greater than 0.90% by weight, achromium (Cr) content of from not smaller than 3.0% by weight to notgreater than 15.0% by weight, a manganese (Mn) content of from notsmaller than 0.10% by weight to not greater than 2.0% by weight, asilicon (Si) content of from not smaller than 0.10% by weight to notgreater than 2.0% by weight, a molybdenum (Mo) content of zero or notgreater than 2.0% by weight; and a vanadium (V) content of zero or notgreater than 2.0% by weight, into a predetermined shape, and thensubjecting the alloy steel to form hardening and tempering.

[0019] The rolling bearing of the present invention can be provided witha prolonged bearing life as compared with rolling bearings formed by theconventional alloy steel (bearing steel such as SUJ2, case hardeningsteel such as SCR420 and SCM420) even when lubricated with a tractionoil and the radius (R) of curvature of the raceway on the inner ring andthe outer ring is from not smaller than 50.1% to not greater than 51.9%of the diameter (D) of the balls (rolling elements) by forming at leastone of the inner ring, the outer ring and the rolling elements formed bythe aforementioned alloy steel.

[0020] The critical significance of the limitation of numeral value ofvarious components of alloy steel will be described below.

[0021] [Carbon (C) Content: From Not Smaller Than 0.50% by Weight to NotGreater Than 0.90% by Weight]

[0022] C is an element which undergoes solid solution in a matrix toprovide the steel with hardness and is bonded to elements such as Cr,Mo, V and W to form a carbide that renders the steel resistant toabrasion. In order to secure hardness and abrasion resistance requiredfor rolling bearing after heat treatment, it is necessary that C isincorporated in an amount of not smaller than 0.50% by weight.

[0023] When C is incorporated in an amount of greater than 0.90% byweight, coarse eutectic carbides can be easily produced during steelmaking, occasionally providing the bearing with drastically deterioratedrolling fatigue life or mechanical strength. Further, cold-workabilityand turnability are deteriorated, adding to work cost.

[0024] C content is preferably from not smaller than 0.55% to notgreater than 0.80%.

[0025] [Chromium (Cr) Content: From Not Smaller Than 3.0% by Weight toNot Greater Than 15.0% by Weight]

[0026] Cr is an element which undergoes solid solution in a matrix toenhance the hardenability, resistance to temper softening, corrosionresistance and other properties thereof and form finely divided carbidesthat prevent the growth of crystalline particles during heat treatmentto prolong the rolling fatigue life or enhance the abrasion resistanceor heat resistance of the matrix. Cr is also an element which stabilizesthe texture to enhance the dimensional stability of the matrix anddrastically inhibit the drop of life due to penetration of hydrogen.When the content of Cr falls below than 3.0% by weight, these effectscannot be sufficiently exerted.

[0027] When the content of Cr exceeds 15.0% by weight, coarse eutecticcarbides can be easily produced during steel making, occasionallyproviding the bearing with drastically deteriorated rolling fatigue lifeor mechanical strength. Further, cold-workability and turnability aredeteriorated, adding to work cost.

[0028] The content of Cr is preferably from not smaller than 4.0% byweight to not greater than 13.5% by weight, more preferably from notsmaller than 4.0% by weight to not greater than 9.0% by weight.

[0029] [Manganese (Mn) Content: From Not Smaller Than 0.10% by Weight toNot Greater Than 2.0% by Weight]

[0030] Mn is an element which acts as a deoxidizer during steel making.Mn also undergoes solid solution in a matrix to enhance thehardenability similarly to Cr. When the content of Mn falls below 0.10%by weight, these effects cannot be substantially exerted.

[0031] When the content of Mn exceeds 2.0% by weight, it is not only tocause the deterioration of cold-workability and turnability but also thedrop of martensite transformation starting temperature that occasionallymakes it impossible to provide a sufficient hardness.

[0032] The content of Mn is preferably from not smaller than 0.10% byweight to not greater than 1.5% by weight.

[0033] [Silicon (Si) Content in Alloy Steel: From Not Smaller Than 0.10%by Weight to Not Greater Than 2.0% by Weight]

[0034] Si is an element which acts as a deoxidizer during steel makingsimilarly to Mn. Si also undergoes solid solution in a matrix to enhancethe hardenability thereof similarly to Cr and Mn. Si is also an elementuseful for strengthening martensite in the matrix to prolong the bearinglife. Si further has an effect of enhancing the temper softeningresistance, dimensional stability and heat resistance of the matrix.When the content of Si falls below 0.10% by weight, these effects cannotbe substantially exerted.

[0035] When the content of Si exceeds 2.0% by weight, thecold-workability, turnability and forgeability of the matrix can bedeteriorated.

[0036] The content of Si is preferably from not smaller than 0.10% byweight to not greater than 1.5% by weight.

[0037] [Molybdenum (Mo) Content in Alloy Steel: From Not Smaller Than 0%by Weight to Not Greater Than 2.0% by Weight]

[0038] Mo is an element which undergoes solid solution in a matrix toenhance the hardenability, resistance to temper softening, corrosionresistance and other properties thereof and form finely divided carbidesthat prevent the growth of crystalline particles during heat treatmentto prolong the rolling fatigue life or enhance the abrasion resistanceor abrasion resistance of the matrix similarly to Cr. Mo is also anelement which stabilizes the texture to enhance the dimensionalstability of the matrix and drastically inhibit the drop of life due topenetration of hydrogen.

[0039] When the content of Mo exceeds 2.0% by weight, coarse eutecticcarbides can be easily produced during steel making, occasionallyproviding the bearing with drastically deteriorated rolling fatigue lifeor mechanical strength. Further, cold-workability and turnability aredeteriorated, adding to work cost.

[0040] Mo is not an essential component of the alloy steel to be used inthe present invention.

[0041] [Vanadium (V) Content in Alloy Steel: From Not Smaller Than 0% byWeight to Not Greater Than 2.0% by Weight]

[0042] V is an element which can easily produce carbides or nitrides.The presence of these carbides or nitrides causes remarkable enhancementof mechanical strength, abrasion resistance and heat resistance of thematrix. V is also an element which stabilizes the texture to enhance thedimensional stability of the matrix and drastically inhibit the drop oflife due to penetration of hydrogen.

[0043] When the content of V exceeds 2.0% by weight, coarse eutecticcarbides can be easily produced during steel making, occasionallyproviding the bearing with drastically deteriorated rolling fatigue lifeor mechanical strength. Further, cold-workability and turnability aredeteriorated, adding to work cost.

[0044] V is not an essential component of the alloy steel to be used inthe present invention.

[0045] The present invention also provides a rolling bearing (radialball bearing) for use in the purpose of bearing the rotary shaft of apulley round which the belt of a belt continuously variable transmissionis wound wherein at least one of the inner ring, the outer ring and therolling elements is formed by the aforementioned specific alloy steel,as the rolling elements there are provided balls, and the radius (R) ofcurvature of the raceway on the inner ring and the outer ring is fromnot smaller than 50.1% to not greater than 51.9% of the diameter of theballs.

[0046] In accordance with this radial ball bearing, the ratio (R/D) isreduced to the same range as disclosed in JP-B-8-30526 to provide astructure such that the ball bearing can difficultly slide in the axialdirection under some thrust load, causing little center dislocationbetween pulleys. Further, since the aforementioned specific alloy steelis used, the bearing life can be prolonged as compared with the casewhere the conventional alloy steels (bearing steel such as SUJ2, casehardening steel such as SCR420 and SCM420) even when lubricated with atraction oil.

[0047] In other words, this radial ball bearing is provided with areduced R/D ratio that reduces center dislocation between pulleys and isformed by the aforementioned specific alloy steel to prolong its life.In this arrangement, the radial ball bearing for supporting the pulleyshaft of a belt continuously variable transmission can exhibit aprolonged bearing life while lessening center dislocation between theboth pulleys round which a belt is wound.

[0048] In the rolling bearing of the present invention, theaforementioned alloy steel preferably has a chromium equivalent of fromnot smaller than 3.5 to not greater than 16.0 as represented by thefollowing equation (1) and the content of retained austenite in thesurface portion thereof after hardening and tempering is from notsmaller than 6% by volume to not greater than 25% by volume:

Chromium equivalent=[Cr]+2[Si]+1.5[Mo]+5[V]  (1)

[0049] wherein [M] represents the content (% by weight) of the elementM.

[0050] By thus predetermining the amount of retained austenite in thesurface portion of the inner ring, the outer ring and the rollingelements to a range of from not smaller than 6% by volume to not greaterthan 25% by volume, surface fatigue can difficultly occur, prolongingthe bearing life, even when foreign matters enter in the gap between theinner ring and outer ring and the rolling elements or slippage occursacross the inner ring and outer ring and the rolling elements.

[0051] When the chromium equivalent falls below 3.5, sufficientdimensional stability cannot be obtained while securing the retainedaustenite content of from not smaller than 6% by volume to not greaterthan 25% by volume. Further, since the rise of Cr, Si, Mo and V contentscauses remarkable deterioration of rolling fatigue or mechanicalstrength or adds to work cost as previously mentioned, the upper limitof chromium equivalent is predetermined to be 16.0. The chromiumequivalent is preferably from 5.0 to 16.0, more preferably from 7.0 to14.0.

[0052] The present invention further provides a belt continuouslyvariable transmission comprising the rolling bearing of the presentinvention by which the rotary shaft of a pulley round which a belt iswound is born.

[0053] In order to accomplish the aforementioned aim, the presentinvention provides a rolling bearing has an inner ring, an outer ring,and a plurality of rolling elements rollably interposed between theinner ring and the outer ring, wherein at least one of the inner ring,the outer ring and the rolling elements is obtained by forming an alloysteel having, a carbon (C) content of from not smaller than 0.10% byweight to not greater than 0.90% by weight, a chromium (Cr) content offrom not smaller than 3.0% by weight to not greater than 8.0% by weight,a manganese (Mn) content of from not smaller than 0.10% by weight to notgreater than 2.0% by weight, and a silicon (Si) content of from notsmaller than 0.10% by weight to not greater than 1.0% by weight into apredetermined shape, and then subjecting the alloy steel to formcarburizing or carbonitriding, hardening and tempering, the totalcontent of carbon and nitrogen in the raceway surface of the ring and/orthe rolling surface of the rolling element is from not smaller than1.20% by weight to not greater than 2.50% by weight, the content ofretained austenite in the raceway surface and/or the rolling surface isfrom not smaller than 15% by volume to not greater than 40% by volume,and the hardness of the raceway surface and/or the rolling surface isfrom not smaller than 59 to not greater than 64 as calculated in termsof Rockwell C hardness (HRC).

[0054] [C+N], retained γ and surface hardness of the aforementionedraceway, etc. represent values on the raceway surface of the ring andthe rolling surface of the finished product which has been subjected togrinding.

[0055] An embodiment of the rolling bearing of the present invention isthe rolling bearing as defined above wherein as the rolling elementsthere are provided balls and the radius of curvature of the raceway onthe inner ring and the outer ring is from not smaller than 50.1% to notgreater than 51.9% (preferably from not smaller than 51.0% to notgreater than 51.9%) of the diameter of the balls.

[0056] The rolling bearing of the present invention can be provided witha prolonged bearing life as compared with rolling bearings formed by theconventional alloy steel (bearing steel such as SUJ2, case hardeningsteel such as SCR420 and SCM420) even when lubricated with a tractionoil and the radius (R) of curvature of the raceway on the inner ring andthe outer ring is from not smaller than 50.1% to not greater than 51.9%of the diameter of the balls (rolling elements) by forming at least oneof the inner ring, the outer ring and the rolling elements formed by theaforementioned alloy steel and predetermining [C+N], retained γ andsurface hardness of the raceway, etc. to the aforementioned range.

[0057] The critical significance of the limitation of numeral value ofvarious components of alloy steel will be described below.

[0058] [Carbon (C) Content: From Not Smaller Than 0.10% by Weight to NotGreater Than 0.90% by Weight]

[0059] C is an element which undergoes solid solution in a matrix toprovide the steel with hardness and is bonded to elements such as Cr,Mo, V and W to form a carbide that renders the steel resistant toabrasion. In order to secure hardness and abrasion resistance requiredfor rolling bearing after heat treatment, it is necessary that C beincorporated in an amount of not smaller than 0.10% by weight. In orderto reduce the carburizing and carbonitriding time and hence inhibit thecost rise, the carbon content is preferably not smaller than 0.50%.

[0060] When C is incorporated in an amount of greater than 0.90% byweight, coarse eutectic carbides can be easily produced during steelmaking, occasionally providing the bearing with drastically deterioratedrolling fatigue life or mechanical strength. Further, cold-workabilityand turnability are deteriorated, adding to work cost.

[0061] C content is preferably from not smaller than 0.55% to notgreater than 0.80%.

[0062] [Chromium (Cr) Content: From Not Smaller Than 3.0% by Weight toNot Greater Than 8.0% by Weight]

[0063] Cr is an element which undergoes solid solution in a matrix toenhance the hardenability, resistance to temper softening, corrosionresistance and other properties thereof and form finely divided carbidesthat prevent the growth of crystalline particles during heat treatmentto prolong the rolling fatigue life or enhance the abrasion resistanceor heat resistance of the matrix. Cr is also an element which stabilizesthe texture to enhance the dimensional stability of the matrix anddrastically inhibit the drop of life due to penetration of hydrogen.When the content of Cr falls below than 3.0% by weight, these effectscannot be sufficiently exerted.

[0064] When the content of Cr is too great, coarse eutectic carbides canbe easily produced during steel making, occasionally providing thebearing with drastically deteriorated rolling fatigue life or mechanicalstrength. Further, cold-workability and turnability are deteriorated,adding to work cost. In particular, when the content of Cr exceeds 8.0%,the resulting alloy steel can be difficultly subjected to carburizingand carbonitriding.

[0065] The content of Cr is preferably from not smaller than 4.0% byweight to not greater than 7.0% by weight.

[0066] [Manganese (Mn) Content: From Not Smaller Than 0.10% by Weight toNot Greater Than 2.0% by Weight]

[0067] Mn is an element which acts as a deoxidizer during steel making.Mn also undergoes solid solution in a matrix to enhance thehardenability similarly to Cr. When the content of Mn falls below 0.10%by weight, these effects cannot be substantially exerted.

[0068] When the content of Mn exceeds 2.0% by weight, it is not only tocause the deterioration of cold-workability and turnability but also thedrop of martensite transformation starting temperature that occasionallymakes it impossible to provide a sufficient hardness.

[0069] The content of Mn is preferably from not smaller than 0.10% byweight to not greater than 1.5% by weight.

[0070] [Silicon (Si) Content in Alloy Steel: From Not Smaller Than 0.10%by Weight to Not Greater Than 1.0% by Weight]

[0071] Si is an element which acts as a deoxidizer during steel makingsimilarly to Mn. Si also undergoes solid solution in a matrix to enhancethe hardenability thereof similarly to Cr and Mn. Si is also an elementuseful for strengthening martensite in the matrix to prolong the bearinglife. Si further has an effect of enhancing the temper softeningresistance, dimensional stability and heat resistance of the matrix.When the content of Si falls below 0.10% by weight, these effects cannotbe substantially exerted.

[0072] When the content of Si exceeds 1.0% by weight, thecold-workability, turnability and forgeability of the matrix can bedeteriorated.

[0073] The content of Si is preferably from not smaller than 0.10% byweight to not greater than 0.50% by weight.

[0074] [Other Alloy Components and Unavoidable Impurities of AlloySteel]

[0075] Oxygen (O) contained in an alloy steel produces an oxide-basedinclusion which causes the deterioration of bearing life. Titanium (Ti)contained in an alloy steel produces a titanium-based inclusion whichcauses the deterioration of bearing life. Therefore, the content of O ispreferably not greater than 10 ppm, and the content of Ti is preferablynot greater than 20 ppm.

[0076] [[C+N] of Raceway, Etc.: From Not Smaller Than 1.20% by Weight toNot Greater Than 2.50% by Weight]

[0077] When [C+N] of the raceway, etc., i.e. the total content of carbonand nitrogen in the raceway surface of the ring (inner ring and/or outerring) and/or the rolling surface of the rolling elements falls below1.20% by weight, surface fatigue can be difficultly lessened whilesecuring sufficient rolling life and heat resistance. Further, when[C+N] exceeds 2.50% by weight, carbides on the grain boundary grow to asize of not smaller than 10 μm or in network, occasionally deterioratingthe rolling life.

[0078] The content of nitrogen in the raceway surface of the ring ispreferably not smaller than 0.10% by weight because some of the carbonatoms can be replaced by nitrogen atoms to eliminate the growth ofcarbides to coarse particles. However, when the content of nitrogenexceeds 0.30% by weight, the grindability, etc. of the product isremarkably deteriorated, adding to cost. In other words, the content ofnitrogen in the raceway surface of the ring is preferably from notsmaller than 0.10% by weight to not greater than 0.30% by weight.

[0079] [Content of Retained Austenite in the Raceway Surface of theRing, Etc.: From Not Smaller Than 15% by Volume to Not Greater Than 40%by Volume]

[0080] Retained austenite has an effect of remarkably lessening surfacefatigue. However, when a rolling bearing for bearing the rotary shaft ofthe pulley of a belt continuously variable transmission has a retainedaustenite content of less than 15% by volume, this effect cannot besufficiently obtained. The content of retained austenite in the racewaysurface of the ring, etc. is preferably not smaller than 20% by volume.

[0081] When the content of retained austenite in the raceway surface ofthe ring, etc. exceeds 40% by volume, the resulting alloy steel exhibitsa lowered surface hardness or undergoes deformation of bearing ringduring assembly. The content of retained austenite in the racewaysurface of the ring, etc. is preferably not smaller than 35% by volume.

[0082] [Surface Hardness of Raceway, Etc.: From 59 to 64 as Calculatedin Terms of HRC]

[0083] When the surface hardness of the raceway, etc. (surface ofraceway and/or rolling surface) falls below 59 as calculated in terms ofRockwell C hardness (HRC), abrasion resistance or surface fatigue cannotbe sufficiently lessened. The surface hardness of the raceway, etc. ispreferably not smaller than 61 as calculated in terms of HRC. The upperlimit of surface hardness is predetermined to 64 taking into accounttoughness.

[0084] The present invention also provides a rolling bearing (radialball bearing) for use in the purpose of bearing the rotary shaft ofpulley round which the belt of a belt continuously variable transmissionis wound wherein at least one of the inner ring, the outer ring and therolling elements is obtained by forming the aforementioned specificalloy steel into a predetermined shape, and then subjecting the alloysteel thus formed to carburizing or carbonitriding, hardening andtempering, the total content of carbon and nitrogen in the racewaysurface of the ring and/or the rolling surface of the rolling element isfrom not smaller than 1.20% by weight to not greater than 2.50% byweight, the content of retained austenite in the raceway surface of thering and/or the rolling surface is from not smaller than 15% by volumeto not greater than 40% by volume, the hardness of the raceway surfaceof the ring and/or the rolling surface is from not smaller than 59 tonot greater than 64 as calculated in terms of Rockwell C hardness (HRC),as the rolling elements there are provided balls and the radius ofcurvature of the raceway on the inner ring and the outer ring is fromnot smaller than 50.1% to not greater than 51.9% of the diameter of theballs.

[0085] In accordance with this radial ball bearing, the ratio (R/D) isreduced to the same range as disclosed in JP-B-8-30526 to provide astructure such that the ball bearing can difficultly slide in the axialdirection under some thrust load, causing little center dislocationbetween pulleys. Further, since the aforementioned specific alloy steelis used and [C+N], retained γ and surface hardness of the raceway, etc.are predetermined to the aforementioned range, the bearing life can beprolonged as compared with the case where the conventional alloy steels(bearing steel such as SUJ2, case hardening steel such as SCR420 andSCM420) even when lubricated with a traction oil.

[0086] In other words, this radial ball bearing is provided with areduced R/D ratio that reduces center dislocation between pulleys and isformed by the aforementioned specific alloy steel and is provided with[C+N], retained γ and surface hardness of raceway predetermined to theaforementioned range to prolong its life. In this arrangement, theradial ball bearing for supporting the pulley shaft of a beltcontinuously variable transmission can exhibit a prolonged bearing lifewhile lessening center dislocation between the both pulleys round whicha belt is wound.

[0087] The present invention further provides a belt continuouslyvariable transmission comprising the rolling bearing of the presentinvention by which the rotary shaft of a pulley round which a belt iswound is born.

BRUEF DESCRIPTION OF THE DRAWINGS

[0088]FIG. 1 is a diagram illustrating the structure of a rollingbearing and the radius of curvature of the raceway of inner and outerrings;

[0089]FIG. 2 is a graph illustrating the relationship between thechromium equivalent of alloy steels constituting the material and thelife ratio (relative value of L10 life) obtained from the results ofembodiment of implementation of the present invention;

[0090]FIG. 3 is a graph illustrating the relationship between thechromium equivalent of alloy steels constituting the material and R/D ofinner ring and life ratio (relative value of L10 life) obtained from theresults of embodiment of implementation of the present invention;

[0091]FIG. 4 is a graph illustrating the relationship between thechromium (Cr) content of alloy steels constituting the material and thelife ratio (relative value of L10 life) obtained from the results ofembodiment of implementation of the present invention;

[0092]FIG. 5 is a graph illustrating the relationship between the totalcontent of carbon (C) and nitrogen (N) in the raceway of the inner ringand outer ring and the life ratio (relative value of L10 life) obtainedfrom the results of embodiment of implementation of the presentinvention;

[0093]FIG. 6 is a sectional view illustrating an example of a beltcontinuously variable transmission for vehicle; and

[0094]FIGS. 7A and 7B are diagrams illustrating how flaking occurs onthe raceway wherein FIG. 7A indicates the case where the lubricant is amineral oil and FIG. 7B indicates the case where the lubricant is atraction oil.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0095] Embodiments of implementation of the present invention will bedescribed hereinafter. TABLE 1 Material Alloying component (wt-%) Cr No.C Si Mn Cr Mo V equivalent A-1 0.50 0.25 1.03 3.05 3.55 A-2 0.90 0.520.51 4.98 6.02 A-3 0.70 0.98 0.46 5.02 6.98 A-4 0.65 0.97 0.78 7.01 8.95A-5 0.68 0.99 0.55 4.01 1.99 8.975 A-6 0.67 0.99 0.49 3.99 1.03 11.12A-7 0.72 0.53 1.05 5.01 0.55 6.895 A-8 0.71 1.02 0.51 8.97 11.01 A-90.69 0.54 0.78 12.99 14.07 A-10 0.58 0.51 0.48 14.90 15.92 B-1 1.01 0.250.31 1.47 1.97 B-2 0.81 0.31 0.46 1.99 2.61 B-3 0.40 0.51 0.55 5.01 6.03B-4 0.99 0.33 0.31 7.00 7.66 B-5 0.71 0.34 0.35 16.01 16.69

[0096] Firstly, alloy steels A-1 to A-10 and B-1 to B-5 comprisingalloying components having the formulation set forth Table 1 above wereprepared. Table 1 also shows Cr equivalent of the various alloy steels.In Table 1, those having an alloying component content falling outsidethe range defined in Claim 1 or Cr equivalent falling outside the rangedefined in Claim 2 are underlined.

[0097] These alloy steels were each used to prepare an inner ring and anouter ring for radial ball bearing (inner diameter: 40 mm; outerdiameter: 80 mm; width: 18 mm) having a nominal count of 6208. Duringthis procedure, the radius Ri of curvature of the raceway 11 of theinner ring 1 and the radius Re of curvature of the raceway 21 of theouter ring 2 shown in FIG. 1 were each determined to various values. Thevarious alloy steels were each formed into a predetermined shape, andthen subjected to hardening at a temperature of from 840° C. to 1,050°C. and tempering at a temperature of 160° C. The alloy steels which hadbeen subjected to heat treatment were each then subjected to grindingand superfinish. The surface roughness of the raceways 11, 21 were eachdetermined to a range of from 0.01 to 0.04 μmRa.

[0098] Separately, balls 3 having a diameter (D) of 11.906 mm made ofsteel corresponding to grade 20 of SUJ2 were prepared. These balls 3 hadbeen subjected to carbonitriding. These balls 3, the aforementionedinner ring 1, the outer ring 2 and a corrugated press retainer made ofmetal (not shown in FIG. 1) were then assembled into test bearings. Thecharacteristics of the various test bearings, i.e., “material”, “surfacehardness of raceway”, “residual γ (retained austenite content) ofraceway”, “radius (R) of curvature of raceway/diameter (D) of ball” ofthe inner ring 1 and the outer ring 2 are set forth in Table 2. Theinner clearance in the radial is referred to as “C3 clearance”.

[0099] 10 samples were prepared for each of these test bearings. Thesesamples were each mounted on a ball bearing life testing machineproduced by NSK Ltd. These samples were each then subjected to life testby rotation under the following conditions.

[0100] Vibration was measured during rotation. When the amplitude ofvibration of the bearing reached five times the initial amplitude ofvibration, rotation was terminated. The rotation time thus reached wasdefined to be life. At this time, the bearing was examined foroccurrence of flaking on the raceway of the inner and outer rings. Inthe case where the amplitude of vibration didn't reach five times theinitial amplitude of vibration, testing was terminated when three timesof 705 hours, which is the calculated life under these conditions,passed.

[0101] Subsequently, the results of test on 10 samples were plotted on aWeibull distribution graph (cumulative failure rate—life) for each ofthese test bearings. From this graph was then determined, beginning withthose having short life, the total rotation time (L10 life) requireduntil 10% of the samples of these test bearings underwent flaking. L10life of the various test bearings were each then calculated relative tothat of No. 25 as 1.

[0102] <Conditions of Life Test>

[0103] Rotary speed: 3,000 rpm

[0104] Radial load: 4,800 N

[0105] Axial load: 2,000 N

[0106] Lubricant: Mixture of a Type NS-1 lubricant for continuouslyvariable transmission (produced by Showa Shell Sekiyu K.K.), which isclassified as “traction oil” with 3% by volume of tap water

[0107] Lubricant temperature: 130° C.

[0108] Rotary ring: Inner ring

[0109] Further, the various test bearings were each kept at 130° C. for1,000 hours for high temperature retention test. The amount ofdimensional change of outer diameter of the bearings from before toafter testing was then measured.

[0110] The results of these tests are set forth in Table 2 below withthe structure of the test bearings. TABLE 2 Constitution of test bearingSurface Results of life test Dimensional hardness retained γ R/D (%) L10life Number of flaking change at high No. Material (Hv) (vol-%) Innerring Outer ring (relative value) Inner ring Outer ring temperature (μm)1 A-1 682 8 50.1 51.0 1.1 8 1 7 2 A-2 745 16 50.1 51.0 1.8 5 0 ≧5 3 A-2745 16 51.0 51.0 2.2 0 3 ≧5 4 A-2 745 16 51.5 51.9 2.7 2 0 ≧5 5 A-3 73812 51.0 51.0 2.7 0 1 ≧5 6 A-4 741 17 50.1 51.0 2.2 3 0 ≧5 7 A-4 741 1751.0 51.5 3.0 0 0 ≧5 8 A-4 741 17 51.9 51.9 3.0 0 0 ≧5 9 A-5 735 14 51.051.0 3.0 0 0 ≧5 10 A-5 735 14 51.9 51.9 3.0 0 0 ≧5 11 A-6 733 10 51.051.9 3.0 0 0 ≧5 12 A-7 732 11 51.0 51.0 2.6 0 2 ≧5 13 A-8 719 19 51.051.0 3.0 0 0 ≧5 14 A-9 731 11 51.0 51.0 3.0 0 0 ≧5 15 A-10 689 25 51.051.0 2.8 2 0 ≧5 16 B-1 733 8 50.1 51.0 0.1 9 1 23 17 B-1 733 8 51.0 51.00.3 2 8 23 18 B-1 733 8 52.0 53.0 0.6 10 0 23 19 B-2 741 12 51.0 51.00.5 1 9 17 20 B-3 603 3 51.0 51.0 0.4 7 3 ≧5 21 B-4 734 16 51.0 51.0 0.28 2 ≧5 22 B-5 722 14 51.0 51.0 0.2 9 1 ≧5 23 A-2 703 5 50.1 51.0 1.2 100 ≧5 24 A-2 703 5 51.0 51.0 1.5 1 9 ≧5 25 A-10 621 34 51.0 51.0 1.0 6 4≧5

[0111] The relationship between the chromium equivalent and the liferatio (relative value of L10 life) of the test bearings (Nos. 3, 5, 7,9, 11 to 15, 19, 22) wherein R/D of the inner ring is 51.0%, the carboncontent in the material used falls within the range defined herein andthe retained γ (retained austenite content) in the raceway falls withinthe range of from 6% to 25% are graphically illustrated in FIG. 2.

[0112] The relationship between the chromium equivalent and R/D of innerring of all the test bearings except No. 18, R/D of inner ring and outerring of which are 52.0% and 53.0%, respectively, are graphicallyillustrated in FIG. 3. In this graph, these plots are represented by “Δ”when the life ratio is not greater than 1.0, “◯” when the life ratio isfrom greater than 1.0 to not greater than 2.0 and “” when the liferatio is greater than 2.0.

[0113] In FIG. 3, the range A indicates a range in which both the tworequirements for (R/D) of inner ring (from not smaller than 50.1% byweight to not greater than 51.9% by weight) and chromium equivalent(from not smaller than 3.5 to not greater than 16.0) are satisfied.There are two “Δ” plots in the range A, but they indicate test bearingscomprising an alloy steel made of alloying components having theformulation falling outside the scope of the present invention.

[0114] As can be seen in these results, the test bearing Nos. 1 to 15have R/D ratio of from not smaller than 50.1% by weight to not greaterthan 51.9% by weight both in the inner and outer rings but are made ofthe alloy steels A-1 to A-10, the alloying components of which fallwithin the scope of the present invention (requirement (1)) and haveretained austenite in the raceway surface of the ring in an amount offrom 6 to 25% by volume (requirement (2)) and a chromium equivalent offrom 3.5 to 16.0 (requirement (3)). Thus, these test bearings canexhibit a prolonged bearing life when lubricated with a traction oil ascompared with the test bearing Nos. 16 to 25, which don't satisfy anyone of the aforementioned requirements (1) to (3).

[0115] In other words, the arrangement of the structures of the testbearing Nos. 1 to 15 can provide a radial ball bearing for bearing thepulley shaft of a belt continuously variable transmission whichundergoes little surface fatigue even with slippage while lesseningcenter dislocation between the two pulleys round which the belt is woundto attain a prolonged bearing life under lubrication with a tractionoil.

[0116] In particular, the test bearing Nos. 2 to 15 exhibited goodresults, i.e., life ratio of not smaller than 1.8 and dimensional changeof not greater than 5 μm at high temperatures because they were made ofthe alloy steels A-2 to A-10, which have a chromium equivalent of fromnot smaller than 5.0 to not greater than 16.0, respectively. On theother hand, the test bearing No. 1 exhibited a dimensional change of 7μm at high temperatures because it was made of an alloy steel having achromium equivalent as relatively low as 3.55. Further, the test bearingNos. 16 to 19 exhibited a dimensional change as great as from 17 μm to23 μm at high temperatures because they were made of alloy steels havinga chromium equivalent as low as 1.97 and 2.61.

[0117] Moreover, the test bearing Nos. 7 to 11, 13 and 14 exhibited goodresults, i.e., life ratio of not smaller than 3.0 because they were madeof the alloy steels A-4 to A-6, A-8 and A-9, which have a chromiumequivalent of from not smaller than 7.0 to not greater than 14,respectively.

[0118] Accordingly, the use of the alloy steels A-2 to A-10, which havealloying components falling within the scope of the present inventionand a chromium equivalent of from not smaller than 5.0 to not greaterthan 16.0, and the predetermination of the content of retained austenitein the raceway surface to a range of from 6 to 25% by volume can providea prolonged bearing life under lubrication with a traction oil and agood dimensional stability at high temperatures while keeping R/D ratioof the inner ring and outer ring to a range of from not smaller than50.1% to not greater than 51.9%.

[0119] In other words, the arrangement of the structures of the testbearing Nos. 2 to 15 can provide a radial ball bearing for bearing thepulley shaft of a belt continuously variable transmission whichundergoes little surface fatigue even with slippage and hence littledimensional change even with heat generation due to slippage whilelessening center dislocation between the two pulleys round which thebelt is wound to attain a prolonged bearing life under lubrication witha traction oil. TABLE 3 Material Alloying component (wt-%) No. C Si MnCr C-1 0.50 0.25 0.78 3.05 C-2 0.55 0.50 0.51 4.01 C-3 0.60 0.47 0.465.02 C-4 0.65 0.30 0.78 5.98 C-5 0.60 0.31 0.55 6.99 C-6 0.80 0.28 0.494.03 C-7 0.90 0.53 0.86 5.01 C-8 0.61 0.99 0.51 4.01 C-9 0.59 0.45 0.788.00 D-1 1.01 0.25 0.31 1.47 D-2 0.75 0.31 0.46 2.01 D-3 0.58 0.48 0.558.95 D-4 0.99 0.33 0.31 6.96 D-5 0.62 1.45 0.35 5.03

[0120] Firstly, alloy steels C-1 to C-9 and D-1 to D-5 comprisingalloying components having the formulation set forth Table 3 above wereprepared. Table 3 also shows Cr equivalent of the various alloy steels.In Table 3, those having an alloying component content falling outsidethe range defined herein are underlined.

[0121] These alloy steels were each used to prepare an inner ring and anouter ring for radial ball bearing (inner diameter: 40 mm; outerdiameter: 80 mm; width: 18 mm) having a nominal count of 6208. Duringthis procedure, the radius Ri of curvature of the raceway 11 of theinner ring 1 and the radius Re of curvature of the raceway 21 of theouter ring 2 shown in FIG. 1 were each determined to various values.

[0122] The various alloy steels were each formed into a predeterminedshape, and then subjected to (C) carburizing or carbonitriding,hardening and tempering or (D) only hardening and tempering (Throughhardening) as heat treatment. In the heat treatment (C), the alloysteels were each heated to a temperature of from 930° C. to 960° C.,subjected to carburizing or carbonitriding for 1 to 3 hours, subjectedto soaking for 1 hour, and then subjected to oil hardening. Temperingwas effected at a temperature of 160° C. In the heat treatment (D), thealloy steels were each subjected to hardening at a temperature of from840° C. to 1,050° C. and tempering at a temperature of 160° C.

[0123] The alloy steels which had been subjected to heat treatment wereeach then subjected to grinding and superfinish. The surface roughnessof the raceways 11, 21 were each determined to a range of from 0.01 to0.04 μmRa.

[0124] Separately, balls 3 having a diameter (D) of 11.906 mm made ofsteel corresponding to grade 20 of SUJ2 were prepared. These balls 3 hadbeen subjected to carbonitriding. These balls 3, the aforementionedinner ring 1, the outer ring 2 and a corrugated press retainer made ofmetal (not shown in FIG. 1) were then assembled into test bearings.

[0125] The characteristics of the various test bearings, i.e.,“material”, “C concentration of raceway (Carbon content-ratio)”, “Nconcentration of raceway (Nitrogen content-ratio)”, “[C+N] of raceway(total content-ratio of Carbon and Nitrogen)”, “surface hardness ofraceway”, “residual γ (retained austenite content) of raceway”, “radius(R) of curvature of raceway/diameter (D) of ball” of the inner ring 1and the outer ring 2 are set forth in Table 4. The inner clearance inthe radial is referred to as “C3 clearance”.

[0126] 10 samples were prepared for each of these test bearings. Thesesamples were each mounted on a ball bearing life testing machineproduced by NSK Ltd. These samples were each then subjected to life testby rotation under the following conditions.

[0127] Vibration was measured during rotation. When the amplitude ofvibration of the bearing reached five times the initial amplitude ofvibration, rotation was terminated. The rotation time thus reached wasdefined to be life. At this time, the bearing was examined foroccurrence of flaking on the raceway of the inner and outer rings. Inthe case where the amplitude of vibration didn't reach five times theinitial amplitude of vibration, testing was terminated when three timesof the calculated life under these conditions, passed.

[0128] Subsequently, the results of test on 10 samples were plotted on aWeibull distribution graph (cumulative failure rate—life) for each ofthese test bearings. From this graph was then determined, beginning withthose having short life, the total rotation time (L10 life) requireduntil 10% of the samples of these test bearings underwent flaking. L10life of the various test bearings were each then calculated relative toaccounting life as 1.

[0129] <Conditions of Life Test>

[0130] Rotary speed: 3,000 rpm

[0131] Radial load: 4,800 N

[0132] Axial load: 2,000 N, 2 Hz ocillating

[0133] Lubricant: Mixture of a Type NS-1 lubricant for continuouslyvariable transmission (produced by Showa Shell Sekiyu K.K.), which isclassified as “traction oil” with 3% by volume of tap water

[0134] Lubricant temperature: 130° C.

[0135] Rotary ring: Inner ring

[0136] The results of these tests are set forth in Table 4 below withthe structure of the test bearings. TABLE 4 Constitution of inner andouter ring of test bearing Surface Surface Results of life test Heatconcentration (wt %) hardness Retained γ R/D (%) L10 life Number offlaking No. Material treatment C N C + N (HRC) (vol-%) Inner ring Outerring (relative value) Inner ring Outer ring 26 C-1 Carburizing 1.22 0.011.23 62.7 32 51.0 51.0 1.8 3 6 27 C-2 Carburizing 1.56 0.01 1.57 63.1 3051.0 51.0 2.4 1 3 28 C-3 Carburizing 2.12 0.01 2.13 63.3 27 51.0 51.92.8 1 0 29 C-4 Carburizing 2.23 0.01 2.24 63.2 25 51.0 51.9 3.0 0 0 30C-5 Carburizing 1.78 0.01 1.79 61.7 15 51.0 51.9 3.0 0 0 31 C-6Carburizing 1.68 0.01 1.69 63.3 28 50.1 51.9 2.1 7 0 32 C-7 Carburizing2.48 0.01 2.49 62.9 24 51.9 51.9 2.2 3 3 33 C-8 Carburizing 1.20 0.011.21 60.9 17 51.9 51.9 2.2 5 2 34 C-9 Carburizing 1.19 0.01 1.20 59.4 1551.0 51.0 1.9 5 2 35 C-1 Carbonitriding 1.13 0.11 1.24 62.9 35 51.0 51.02.0 1 5 36 C-2 Carbonitriding 1.45 0.17 1.62 63.4 33 51.0 51.0 2.6 0 237 C-3 Carbonitriding 1.89 0.19 2.08 64.0 28 51.0 51.0 3.0 0 0 38 C-4Carbonitriding 2.11 0.18 2.29 63.4 27 51.0 51.0 3.0 0 0 39 C-5Carbonitriding 1.75 0.22 1.97 62.8 23 51.0 51.0 3.0 0 0 40 C-6Carbonitriding 1.55 0.14 1.69 63.8 30 51.0 51.0 2.9 0 1 41 C-7Carbonitriding 2.28 0.19 2.47 63.3 28 51.0 51.0 2.7 1 1 42 C-8Carbonitriding 1.19 0.29 1.48 61.9 25 51.0 51.0 2.4 1 2 43 C-9Carbonitriding 1.21 0.22 1.43 60.1 18 51.0 51.0 2.5 1 2 44 D-1 Through1.04 — 1.04 61.4 8 52.0 52.0 0.4 1 9 hardening 45 D-1 Through 1.04 —1.04 61.4 8 50.1 51.0 0.2 10 0 hardening 46 D-1 Through 1.04 — 1.04 61.48 53.0 54.0 0.2 8 2 hardening 47 D-2 Carburizing 1.20 0.01 1.21 62.9 3351.0 51.0 0.8 1 9 48 D-3 Carburizing 0.67 0.01 0.68 61.2 18 51.0 51.00.8 1 9 49 D-4 Carburizing 1.91 0.01 1.92 61.9 17 51.0 51.0 0.5 4 6 50D-5 Carburizing 0.83 0.01 0.84 62.1 18 51.0 51.0 0.9 2 8 51 D-2Carbonitriding 1.17 0.13 1.30 63.2 35 51.0 51.0 0.9 1 9 52 D-3Carbonitriding 0.64 0.08 0.72 61.3 17 51.0 51.0 0.9 1 9 53 D-4Carbonitriding 1.78 0.16 1.94 62.2 19 51.0 51.0 0.6 4 6 54 D-5Carbonitriding 0.79 0.09 0.88 62.0 17 51.0 51.0 0.9 2 8

[0137] The relationship between the chromium content in the alloy steelsused in the inner ring and outer ring of the various test bearings andthe resulting life ratio (relative value of L10 life) of these testbearings are graphically illustrated in FIG. 4. The relationship betweenthe total content of C and N in the raceway of the inner ring and outerring of the various test bearings and the resulting life ratio (relativevalue of L10 life) of these test bearings are graphically illustrated inFIG. 5.

[0138] In FIG. 4, the range H1 indicates a range in which the content ofCr in the alloy steel falls within the range defined in the presentinvention (3.0 to 8.0% by weight). In FIG. 5, the range H2 indicates arange in which the total content of C and N in the raceway surface ofthe ring falls within the range defined in the present invention (1.20to 2.50% by weight).

[0139] Further, the test bearings were kept at a temperature of 130° C.for 1,000 hours or more. The outer diameter of the bearings weremeasured before and after aging to measure the dimensional changethereof. The test bearing Nos. 26 to 43, which correspond to examples ofthe present invention, exhibited a negligibly small dimensional change.

[0140] As can be seen in these results, the test bearing Nos. 26 to 43have R/D ratio of from not smaller than 50.1% by weight to not greaterthan 51.9% by weight both in the inner and outer rings but are made ofthe alloy steels C-1 to C-10, the alloying components of which fallwithin the scope of the present invention (requirement (1)), have C andN incorporated in the raceway surface of the ring in a total amount offrom not smaller than 1.20% by weight to not greater than 2.50% byweight (requirement (2)) and retained austenite in the raceway surfaceof the ring in an amount of from 15 to 40% by volume (requirement (3))and exhibit a raceway surface hardness of from not smaller than 59 tonot greater than 64 as calculated in terms of HRC (requirement (4)).Thus, these test bearings can exhibit a prolonged bearing life whenlubricated with a traction oil as compared with the test bearing Nos. 44to 54, which don't satisfy any one of the aforementioned requirements(1) to (3).

[0141] Thus, the arrangement of the structures of the test bearing Nos.26 to 43, the inner ring and outer ring of which fall within the scopeof the present invention, can provide a radial ball bearing for bearingthe pulley shaft of a belt continuously variable transmission whichundergoes little surface fatigue even with slippage while lesseningcenter dislocation between the two pulleys round which the belt is woundto attain a prolonged bearing life under lubrication with a tractionoil.

[0142] In other words, in accordance with the present invention, the useof an alloy steel having a high chromium content and the enhancement of[C+N] in the raceway by carburizing or carbonitriding cause enhancementof texture stability and surface fatigue resistance, resulting in theprolongation of the life of rolling bearings which are used inenvironments subject to slippage. Further, the arrangement of thecontent of retained austenite in the raceway surface of the ring to apredetermined range makes it possible to secure desired heat resistanceand dimensional stability even with increased generation of heat due toslippage and hence prolong life against seizing.

[0143] In particular, the test bearing Nos. 35 to 43 exhibited a racewaynitrogen concentration of from not smaller than 0.1% by weight to notgreater than 0.3% by weight due to carbonitriding and thus exhibited ahigher life ratio than the test bearing Nos. 35 to 44, which had beensubjected to carburizing.

[0144] The test bearing Nos. 44 to 46 were obtained by subjecting D-1,which corresponds to the related art bearing steel, to Throughhardening. These test bearings had different R/D ratios of inner ringand outer ring. The test bearing No. 45, which has an inner ring R/Dratio of 50.1%, and the test bearing No. 46, which has an inner ring R/Dratio of 53.0%, exhibited a shorter L10 life than the test bearing No.45, which has an inner ring R/D ratio of 52.0% (0.2 times the calculatedlife). As can be seen in these results, in the case where the relatedart bearing steels are used, when R/D is predetermined to a range offrom not smaller than 50.1% to not greater than 51.9%, the bearing lifeis reduced.

[0145] As mentioned above, in accordance with the present invention, theuse of a specific alloy steel makes it possible to prolong the bearinglife as compared with rolling bearings formed by the conventional alloysteels (bearing steel such as SUJ2, case hardening steel such as SCR420and SCM420) even when lubricated with a traction oil and the radius (R)of curvature of the raceway on the inner ring and the outer ring is fromnot smaller than 50.1% to not greater than 51.9% of the diameter (D) ofthe balls.

[0146] In some detail, the predetermination of the radius (R) ofcurvature of the raceway of inner and outer rings to a range of from notsmaller than 50.1% to not greater than 51.9% of the diameter (D) of theballs and the use of a specific alloy steel make it possible to providea radial ball bearing for bearing the pulley shaft of a beltcontinuously variable transmission which can exhibit a prolonged bearinglife while lessening center dislocation between the two pulleys roundwhich the belt is wound.

[0147] Further, the arrangement of the structure of the presentinvention makes it possible to exert an effect of causing little surfacefatigue even with slippage and hence little dimensional change with heatgeneration due to slippage, resulting in difficulty in occurrence ofseizing, in addition to the aforementioned effect.

[0148] Moreover, in accordance with the belt continuously variabletransmission of the present invention, the rotary shaft of the pulleyround which the belt is wound is born by the rolling bearing of thepresent invention, making it possible to rotate the pulleys stably overan extended period of time while lessening center dislocation betweenthe two pulleys and hence keeping the belt durable over an extendedperiod of time.

[0149] As mentioned above, in accordance with the present invention, theuse of a specific alloy steel and the arrangement of [C+N], retained γand surface hardness of the raceway surface of the ring to predeterminedrange make it possible to lessen heat generation or surface fatigue dueto metallic contact and make it difficult to produce a newly producedsurface on the raceway. Thus, the bearing life can be prolonged ascompared with rolling bearings formed by the conventional alloy steels(bearing steel such as SUJ2, case hardening steel such as SCR420 andSCM420) even when lubricated with a traction oil and the radius (R) ofcurvature of the raceway on the inner ring and the outer ring is fromnot smaller than 50.1% to not greater than 51.9% of the diameter of theballs.

[0150] In some detail, the predetermination of the radius (R) ofcurvature of the raceway of inner and outer rings to a range of from notsmaller than 50.1% to not greater than 51.9% of the diameter (D) of theballs, the use of a specific alloy steel and the arrangement of [C+N],retained γ and surface hardness of the raceway surface of the ring topredetermined range make it possible to provide a radial ball bearingfor bearing the pulley shaft of a belt continuously variabletransmission which can exhibit a prolonged bearing life while lesseningcenter dislocation between the two pulleys round which the belt iswound.

[0151] Further, in accordance with the belt continuously variabletransmission of the present invention, the rotary shaft of the pulleyround which the belt is wound is born by the rolling bearing of thepresent invention, making it possible to rotate the pulleys stably overan extended period of time while lessening center dislocation betweenthe two pulleys and hence keeping the belt durable over an extendedperiod of time.

What is claimed is:
 1. A rolling bearing comprising: an inner ring; anouter ring; a plurality of balls as rolling elements rollably interposedbetween the inner ring and the outer ring, wherein the radius ofcurvature of the raceway on the inner ring and the outer ring is fromnot smaller than 50.1% to not greater than 51.9% of the diameter of theballs, and at least one of the inner ring, the outer ring and therolling elements is obtained by forming an alloy steel having: a carbon(C) content of from not smaller than 0.50% by weight to not greater than0.90% by weight; a chromium (Cr) content of from not smaller than 3.0%by weight to not greater than 15.0% by weight; a manganese (Mn) contentof from not smaller than 0.10% by weight to not greater than 2.0% byweight; a silicon (Si) content of from not smaller than 0.10% by weightto not greater than 2.0% by weight; a molybdenum (Mo) content of zero ornot greater than 2.0% by weight; and a vanadium (V) content of zero ornot greater than 2.0% by weight, into a predetermined shape, and thensubjecting the alloy steel to form hardening and tempering.
 2. Therolling bearing according to claim 1, wherein the alloy steel has achromium equivalent of from not smaller than 3.5 to not greater than16.0 as represented by the following equation (1) and the content ofretained austenite in the surface portion thereof after hardening andtempering is from not smaller than 6% by volume to not greater than 25%by volume: Chromium equivalent=[Cr]+2[Si]+1.5[Mo]+5[V]  (1) wherein [M]represents the content (% by weight) of the element M.
 3. A rollingbearing comprising: an inner ring; an outer ring; and a plurality ofrolling elements rollably interposed between the inner ring and theouter ring, wherein at least one of the inner ring, the outer ring andthe rolling elements is obtained by forming an alloy steel having: acarbon (C) content of from not smaller than 0.10% by weight to notgreater than 0.90% by weight; a chromium (Cr) content of from notsmaller than 3.0% by weight to not greater than 8.0% by weight; amanganese (Mn) content of from not smaller than 0.10% by weight to notgreater than 2.0% by weight; and a silicon (Si) content of from notsmaller than 0.10% by weight to not greater than 1.0% by weight into apredetermined shape, and then subjecting the alloy steel to formcarburizing or carbonitriding, hardening and tempering, the totalcontent of carbon and nitrogen in the raceway surface of the ring and/orthe rolling surface of the rolling element is from not smaller than1.20% by weight to not greater than 2.50% by weight, the content ofretained austenite in the raceway surface and/or the rolling surface isfrom not smaller than 15% by volume to not greater than 40% by volume,and the hardness of the raceway surface and/or the rolling surface isfrom not smaller than 59 to not greater than 64 as calculated in termsof Rockwell C hardness (HRC).
 4. The rolling bearing according to claim3, wherein balls as the rolling elements are provided and the radius ofcurvature of the raceway on the inner ring and the outer ring is fromnot smaller than 50.1% to not greater than 51.9% of the diameter of theballs.
 5. The rolling bearing according to claim 1, which is used forthe purpose of bearing a rotary shaft of a pulley, a belt of a beltcontinuously variable transmission being wound around the pulley.
 6. Therolling bearing according to claim 2, which is used for the purpose ofbearing a rotary shaft of a pulley, a belt of a belt continuouslyvariable transmission being wound around the pulley.
 7. The rollingbearing according to claim 4, which is used for the purpose of bearing arotary shaft of a pulley, a belt of a belt continuously variabletransmission being wound around the pulley.
 8. A belt continuouslyvariable transmission comprising a rolling bearing according to claim 1by which a rotary shaft of a pulley is born, a belt being wound aroundthe pulley.
 9. A belt continuously variable transmission comprising arolling bearing according to claim 2 by which a rotary shaft of a pulleyis born, a belt being wound around the pulley.
 10. A belt continuouslyvariable transmission comprising a rolling bearing according to claim 4by which a rotary shaft of a pulley is born, a belt being wound aroundthe pulley.